1. Field of the Invention
This invention relates to a process for the alkylation of an aromatic hydrocarbon by reaction with an olefin in the presence of a crystalline aluminosilicate catalyst which has undergone prior modification by subjection to a thermal treatment.
2. Description of the Prior Art
Alkylation of aromatic hydrocarbons utilizing crystalline aluminosilicate catalysts has heretofore been described. U.S. Pat. No. 2,904,607 to Mattox refers to alkylation of aromatic hydrocarbons with an olefin in the presence of a crystalline metallic aluminosilicate having uniform pore openings of about 6 to 15 Angstrom units. U.S. Pat. No. 3,251,897 to Wise describes liquid phase alkylation in the presence of X- or Y-type crystalline aluminosilicate zeolites, specifically such type zeolites wherein the cation is rare earth and/or hydrogen. U.S. Pat. No. 3,751,504 to Keown et al., and U.S. Pat. No. 3,751,506 to Burress describe vapor phase alkylation of aromatic hydrocarbons with olefins, e.g. benzene wth ethylene, in the presence of a ZSM-5 type zeolite catalyst.
While the latter type catalysts represent a distinct improvement over previously suggested crystalline aluminosilicate catalysts particularly with respect to improved aging properties, they have the disadvantage of producing unwanted quantities of impurities along with the desired alkyl aromatic product, thereby decreasing the overall yield and selectivity for such product.
Thus, in the alkylation of benzene with ethylene, while desired ethylbenzene is the major product, small amounts of di- and possibly triethylbenzenes are always produced simultaneously with ethylbenzene, such amounts depending on the conversion of benzene to ethylbenzene. The polyethylbenzenes formed can be recycled to the alkylation zone, where they undergo transalkylation with benzene to produce more ethylbenzene. Alternatively, the polyethylbenzenes can be transalkylated with benzene in a separate reactor. The formation of polyethylbenzenes hence does not constitute an ultimate loss of the alkylating agent, ethylene. On the other hand, aromatic compounds other than ethyl- and polyethylbenzenes, that are formed during the alkylation reaction, generally referred to as by-products, result in an irreversible loss of ethylene and cause difficulties in the product purification. By-products produced during ethylation of benzene include, for example, toluene, xylenes, cumene, n-propylbenzene, ethyltoluene, butylbenzene and other C.sub.10 + aromatics, the majority being C.sub.7 -C.sub.9 aromatics. The formation of these by-products is increased when the benzene conversion to ethylbenzene is high. Due to the high exothermicity of the alkylation reaction, ethylbenzene synthesis is generally carried out in a multiplicity of reactors with interstage cooling and addition of ethylene to the various stages, the ethylbenzene concentration increasing in subsequent stages. Undesired by-products are accordingly formed in increasing amounts in the latter stages of the process.